The 2005 Major League Baseball season was notable for the league's new steroidpolicy in the wake of the BALCO scandal, which enforced harsher penalties ever than before for steroid use in Major League Baseball. Several players, including veteran Rafael Palmeiro, were suspended under the new policy. Also, every team in the NL East division had at least 81 wins (at least half of the 162 games played). It was also notable for being the first season featuring a baseball team in Washington, DC for more than 4 decades, with the Washington Nationals having moved from Montreal.

The standings in the National League East were quite notable because all the teams in that division finished with at least a .500 record. The San Diego Padres' record of 82-80 was also notable as it was the worst ever by a division champion in a full-length season. (It is not, strictly speaking, the worst record by a division champion as the strike-shortened 1994 season ended with every team in the AL West at least 10 games under .500, nor is it the worst record by a team that made the postseason as due to the "split-season" format imposed in the wake of the 1981 Major League Baseball strike, the Kansas City Royals went to the playoffs as second-half AL West champions despite an overall record of 50-53.)

†Denotes the club that won the wild card for its respective league.
‡The Yankees and Red Sox finished with the same win-loss record, however because the Yankees had a better head-to-head result against the Red Sox and because the teams' records qualified for the wild card, a one game playoff was not played - instead, the division championship was awarded to the Yankees and the wild card to the Red Sox.

Click on any series score to link to that series' page.
Higher seed had home field advantage during Division Series and League Championship Series.
The American League champion had home field advantage during the World Series as a result of the AL victory in the 2005 All-Star Game.